Photo by John Burns


Severity is the quantity of disease affecting the sampling unit. For corals, severity is the quantity of disease affecting an individual coral colony or a group of surveyed colonies. Severity measures are typically expressed as a proportion or percentage of the sampling unit that is affected by a disease or reduced health state. For example, if 3/4 of a colony surface area is affected by a certain disease then the severity would be expressed as 75%. 

Severity gives a more accurate measure of the extent to which a disease impacts an affected population since is takes relative cover into account, rather than simple presence or absence of a condition.